This invention relates to improvements in a power steering apparatus for use in a vehicle having steerable vehicle wheels.
During operation of a vehicle at relatively low speeds, such as during parking, there is a relatively large resistance to turning movement of steerable vehicle wheels. Therefore, relatively high fluid pressures are desirable to actuate a power steering motor at low vehicle speeds. As vehicle speed increases, the resistance to turning movement of the steerable vehicle wheels decreases. Therefore, the fluid pressure required to actuate the power steering motor decreases as vehicle speed increases. During operation of a vehicle at relatively high speeds, the assistance provided by a power steering motor should be minimized to reduce energy requirements, prevent excessive turning movement of the vehicle wheels in response to small movements of the steering wheel and to provide the operator of the vehicle with a feel for road conditions.
When an operator rotates a steering wheel at low vehicle speeds, the power steering system should respond immediately. If there is a delay in the response of the power steering system to rotation of the steering wheel, the operator will initially feel a substantial resistance to turning of the steering wheel. This initial resistance to steering wheel rotation will disappear as the power steering system becomes effective. The resulting jerkiness in steering wheel rotation will be objectionable and disconcerting to the operator.
Power steering systems in automotive vehicles commonly have an engine driven pump. In power steering systems, maximum steering assist and hence maximum flow from the pumps is required at low vehicle and minimum engine speeds. Thus, a power steering pump requires substantial power at low engine speeds. If the vehicle has a small engine, the load applied to the engine by the power steering pump during low speed operation of the vehicle may cause the engine to stall. Normally, flow is related to pump speed. The product of flow, pressure and speed is related to power. Hence, as pump speed increases, the power required to drive the pump also increases. The additional power must be supplied by the engine and hence results in wasted power consumption and reduced fuel economy. Hence, as pump speed increases, flow also increases. However, since the demand for flow is maximum at low vehicle and engine speeds, the increased flow is superfluous to proper steering assist and is wasted. Thus, during high speed operation of the vehicle, the engine driven power steering pump increases the energy (fuel) required to operate the vehicle.